comparator oscillator

Discussion in 'The Projects Forum' started by tibbles, Jul 24, 2014.

  1. tibbles

    Thread Starter Active Member

    Jun 27, 2008
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    3
    hi
    following on from my previous post, re maths, i spent some hours yesterday,trying to get the circuit- http://www.slottechforum.com/slotin...ltage Comparators/LM339/LM339 oscillators.pdf bottom left, to work, without success, i tried two more similar circuits, again unsuccessfully,the circuits showed either no oscillation,or unstable oscillations at many different frequencies, none anywhere close to the xtal freqency of two megahertz
    i checked each board twice for errors, none were found, the circuits are fairly simple, so not much to go wrong.
    i am using the LM319N high speed comparator which should be ok at this frequency, i think.
    i tried the xtal in a transistor based oscillator and it worked fine,
    ive decided to weaken and go with a can type xtal oscillater,but id love to know why these fairly generic circuits didnt work.i must have missed something drastic,
    any help much appreciated
    doug
     
  2. panic mode

    Senior Member

    Oct 10, 2011
    1,318
    304
    what are the values for R12 and C1?
     
  3. tibbles

    Thread Starter Active Member

    Jun 27, 2008
    249
    3
    hi
    just spotted a post on decoupling caps, that could be the problem. i have been using a 10 n across the power,as it was the first one that came to hand,so with the lack of any replies, i might try doing it properly.
    any suggestions for values welcome-2 mhz.
    it would also be helpful to check on the speed of the LM319 it gives this value in nanoseconds, it would be helpful to know how this equates to mhz,
    the can oscillater arrives tomorrow, ive never used one before, so it will be interesting to see how it performs,
    sorry to be a pain
    doug
     
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,969
    744
    i think it will only work upto 1Mhz, try the circuit at the top left, using 50pF for C1, 100K for R12, then reduce R12 in 10K steps and see when it stops oscillating.

    Try using a CA3240 op amp instead
     
    Last edited: Jul 24, 2014
  5. tibbles

    Thread Starter Active Member

    Jun 27, 2008
    249
    3
    2Mhz is a 10K resistor and 50pF cap.according to the equation.
    thanks
    doug
     
  6. tibbles

    Thread Starter Active Member

    Jun 27, 2008
    249
    3
    dodgydave ,thanks for coming to my rescue again, however i think poverty is the mother of invention as well, dont laugh- go on then. but back in the day these were very expensive, and i had plenty of copper pipe and clips
     
  7. tibbles

    Thread Starter Active Member

    Jun 27, 2008
    249
    3
    just found the convertor -1,000 / ns. the 319 has a rise time of 80 ns,
    so if ive done my maths correctly, thats 12.5 mhz,
     
  8. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,282
    1,233
    That is a thing of beauty!:eek:
     
  9. panic mode

    Senior Member

    Oct 10, 2011
    1,318
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    yea? i wouldn't be so sure.... how about fall time?
    to make a one period you need both rising and falling edge.

    160ns means 6.25MHz and that is with absolutely no load, capacitance or anything else (pure propagation). the moment you add other components, this "6MHz" melts away... ;)
     
  10. tibbles

    Thread Starter Active Member

    Jun 27, 2008
    249
    3
    the advice was spot on,
    progressivly reduced R AND C, all went well up to 500 khz
    then reducing RC further made no difference,
    at least we've got to the bottom of it.
    thank you all
    regards
    doug
     
    Dodgydave likes this.
  11. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,775
    1,103
    Love all the plumbing in post #6. But with today's copper prices it might have been cheaper to buy a drill-stand :D.
     
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